Polyphenylene ether resins are known in the art and exhibit a desirable combination of chemical, physical and electrical properties over a temperature range of more than about 650.degree. F., extending from a brittle point of about -275.degree. F. to a heat distortion temperature of about 375.degree. F. This combination of properties renders polyphenylene ether resins suitable for a broad range of applications. However, the usefulness of polyphenylene ether resins is limited in some applications as a consequence of processability, impact resistance and chemical resistance. It is known to modify polyphenylene ether resins with various compounds in order to improve the processability and the impact and chemical resistances of the resins. Alternatively, polyphenylene ether resins have been blended with other resins in order to improve their undesirable properties.
However, as is well known in the polymer art, blending of polymers often leads to disappointing results. Most polymers are not actually soluble in one another, and even if they are intimately mixed by mechanical means, the existence of phase boundaries between the components can result in poor overall physical and mechanical properties. For example, when polyphenylene ethers are blended with linear polyesters, such as polybutylene terephthalate, which are known to have good chemical resistance, the resulting blends generally are of poor quality owing to the incompatibility of the polyphenylene ether and the polyester. The blends exhibit poor interfacial adhesion between the component polymer phases and thus poor mechanical properties, as well as poor surface appearance and low heat distortion temperatures.
Various blending strategies which have been employed in the art include the use of plasticizing additives and styrene-butadiene copolymer resins as taught by the Lee U.S. Pat. No. 4,123,410. Lee discloses the use of large amounts of the plasticizer, preferably 10 to 15 percent, which results in a loss of heat distortion temperature. Such compositions including the plasticizer are also less apt to retain their properties when exposed to solvents, since the plasticizer may be extracted by the solvent.
Blending of polyphenylene ethers with polyesters has also been accomplished by including phenoxy resins in combination with the polymers as disclosed in European patent application No. 148,774. However, the resulting blends are quite brittle. The phenoxy resin content also may be expected to lessen the solvent resistance of the blend.
It is an object of the present invention to provide blend compositions of polyphenylene ether and a polyester such as a linear polyester, which compositions exhibit good chemical and solvent resistance and good flowability during molding, as well as good mechanical and thermal properties of the molded product.
It is a further object of the present invention to provide a process which facilitates the blending of a polyphenylene ether with a polyester such as polybutylene terephthalate. PA1 These and additional objects are provided by the present invention which comprises blend compositions of a polyphenylene ether and a property-improving amount of a polyester which has been modified with a hydroxy- or carboxy-terminated polystyrene. The blend compositions have been found to exhibit good flow properties in molding, together with good chemical solvent resistance and mechanical properties of the molded plastic. Particularly favorable results are obtained when the polyester is a linear polyester such as a polyalkylene terephthalate, for example, polybutylene terephthalate. These and additional objects and advantages will be more fully understood in view of the following detailed description.